Conventional low density polyethylene (LDPE) has good processability; however, when used in film and/or extrusion coating application, increased melt strength is still desired.
U.S. Publication No. 2008/0242809 and International Publication Nos. WO 2007/110127, WO97/45465, WO 2012/057975 and WO 2012/084787 describe using various multifunctional components to broaden molecular weight distribution (MWD) and/or modify the rheological properties for making a copolymer suitable for extrusion coating applications, among others. The multifunctional components include, among others, di- and/or higher functional (meth)acrylates, a bifunctional α,ω-alkadienes, diunsaturated comonomers containing divinyl ether and monomeric chain transfer agents (CTAs).
International Application No. PCT/US13/029881 (now WO 2014/03837) discloses an ethylene-based polymer formed from reacting ethylene and at least one asymmetrical polyene comprising an “alpha, beta unsaturated end” and a “C—C double bond end,” wherein the reaction takes place in the presence of at least one free-radical initiator.
The impact of above described multifunctional components on the final polymer through coupling and/or branching reactions is complex, and depends on the type and reactivity of the functional groups. A vinyl functional group will act as a comonomer and incorporate into a polymer chain/molecule. When involved, CTA functionality will either start the formation of a new polymer molecule, or initiate, after incorporation of the monomeric group, the formation of a long chain branch (LCB) or T-branch. For a multi- and/or bifunctional component to impact polymer rheology, it is important that (1) at least two functional groups of the component react and (2) effective branches are formed in the polymer.
H-branches are either intermolecular (between two molecules) or intra-molecular (within a molecule) and formed by reaction of two or more vinyl groups of the bi- and/or multifunctional component. The probability that functional groups will react, and contribute to a melt strength increase, depends on the reactivity of the functional groups, overall and remaining conversion level, and molecular topology of the polymer, showing how the component is incorporated by its first reacting functionality. The impact of H-branch formation on melt strength will be (1) negligible with intra-molecular H-branch formation, (2) low for intermolecular H-branch formation between two small polymer molecules, and (3) significant for intermolecular H-branch formation between two larger molecules. However, the latter (3) could lead to the formation of gels, especially when crosslinked networks are formed between and inside large polymer molecules.
Taking into account the reaction kinetic data reported by Ehrlich and Mortimer in Adv. Polymer Sci., Vol 7, pp. 386-448 (1970), and a typical ethylene conversion level in a tubular reactor of 25-35%, the following general remarks can be made: (i) the incorporation level per reactor pass is less than 50% for hydrocarbon dienes, while the probability of forming H-branches is less than 10%; (ii) a monomeric CTA containing acrylate monomer functionality will have a high incorporation level per reactor pass, but further reaction would be required to form a T-branch; and (iii) the probability that the CTA functionality will react, depends on chain transfer activity and remaining conversion level. For compounds with a CTA functionality similar to typically used CTAs for the high pressure LDPE process, the amount of T-branching formed would be low. Di- or higher functional (meth)acrylate components will lead to almost complete incorporation in the polymer, and a very high level of secondary reaction. The high reactivity of the functional groups makes even distribution over the polymer formed in a tubular reactor difficult. Furthermore, coupling or H-branch formation, when the component is fed to the first reaction zone will already occur in the first reaction zone, thus increasing the risk of initiating and/or forming product gels and fouling in the first reaction zone, with further exposure and deterioration in the remaining reaction/cooling zones if present.
Various publications describe methods for modeling and/or simulating polymerizations using branching agents and predicting the properties of resulting polymers. Such publications include, but are not limited to, Liu, J., et al., Branched Polymer via Free Radical Polymerization of Chain Transfer Monomer: A Theoretical and Experimental Investigation, J. Polym. Sci. Part A: Polym. Chem., (2007), 46, 1449-59; Wu, P-C et al, Monte Carlo simulation of structure of Low-Density polyethylene; Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No 3, 352-357 (1972); Iedema, P. D. et al., Rheological Characterization of Computationally Synthesized Reactor Populations of Hyperbranched Macromolecules; Bivarate Seniority-Priority Distribution of IdPE, Macromolecular Theory and Simulations, 13, 400-418 (2004); T. C. B. McLeish et al, Molecular rheology of H-Polymers, Macromolecules, 21, 1062-1070 (1988); and D. J. Read et al, Linking models of polymerization and dynamics to predict branched polymer structure and flow; Science, 333, 1871-1874 (2011).
The continuous stirred tank reactor (CSTR), or autoclave, process typically leads to more Cayley tree structured molecular polymer topology, due to the inherent more homogeneous LCB level and chain segment size distribution, while the residence time distribution creates very long and very short growth paths, leading to a broader MWD. Furthermore, a comonomer will be homogenously incorporated within a reaction zone regardless of its reactivity. The tubular reactor process typically leads to more comb-shaped molecular polymer topology, due to the low starting LCB level and the lower temperature conditions leading to long chain segments, while the MWD is narrowed, due to the more homogenous residence time distribution. However, the lack of back mixing, as present in a CSTR reactor, or axial mixing, leads to a comonomer incorporation distribution that is strongly affected by the reactivity of the comonomer and the changing composition of reactants along the tubular reactor. Incorporation in a larger polymer molecule, at a position more inside the polymer molecule sphere (higher priority and seniority in Cayley tree structure), may affect reactivity and increase the probability for an intra-molecular reaction. Incorporation in smaller (lower gyration radius) and/or linear polymer molecules (comb like structure) and/or at a position more at the outer side of the polymer molecule sphere (lower priority and seniority in a Cayley tree structure), may affect the reactivity less, and increase the probability for an intermolecular reaction.
International Publication No. WO 2013/059042 describes using fresh ethylene and/or CTA feed distributions to broaden MWD and increase melt strength, while remaining process conditions are constant. International Publication No. WO 2013/078018 describes low density ethylene-based polymers with broad MWDs and low extractables made in a tubular reactor in the absence of an added crosslinking agent and/or comonomer with crosslinking capability.
International Publication No. WO 2013/078224 describes broad MWD tubular LDPE resins with low extractables. The polymerization conditions need to be carefully selected and balanced to reduce extraction at higher molecular weights. Important process parameters include maximum polymerization temperatures, reactor pressure, and the type, level and distribution of the CTA.
There remains a need for new ethylene-based polymers that have higher melt strengths at high and low densities, and which can be made in a tubular reactor at low gel levels. There is a further need for such polymers that also have lower n-hexane extractable content. These needs have been met by the following invention.